In a photolithography process of semiconductor manufacturing processes, a photomask is used. With the miniaturization of semiconductor devices, a demand for miniaturization in this photolithography process has been increasing. Particularly, an increase in NA of an exposure apparatus using ArF exposure light (193 nm) has proceeded for adaptation to the miniaturization and a further increase in NA is proceeding following the introduction of the immersion exposure technique. For adaptation to the demand for the high miniaturization and the increase in NA described above, it is required to enhance the flatness of a photomask. That is, in view of the fact that the allowable width of position offset of a transfer pattern due to the flatness has been reduced with the reduction in pattern line width and that the focal depth in the photolithography process has been reduced with the increase in NA, the flatness of main surfaces of a mask substrate, particularly the main surface on the side where a pattern is to be formed (hereinafter, the main surface on this side will be referred to simply as a main surface or a substrate main surface), is becoming unignorable.
FIG. 6 is diagrams showing the shapes of a substrate of a photomask before (before suction) and after (after suction) the photomask is chucked in an exposure apparatus, wherein FIG. 6(a) is a diagram showing the shape of the substrate before suction while FIG. 6(b) shows the shape of the substrate after suction. As seen from FIG. 6(a), four corners of the substrate are a little higher than chuck areas of a main surface and the height gradually increases toward its central portion. That is, generally circular contour lines are shown in the substrate before suction. In the substrate after suction, generally rectangular contour lines are shown as seen from FIG. 6(b). In this manner, when the photomask is chucked on a mask stage of the exposure apparatus by a vacuum chuck, it may happen that the photomask is largely deformed upon chucking due to the affinity with the mask stage or the vacuum chuck.
Conventionally, since the product management is conducted in terms of the flatness of the photomask before chucking, it may happen that even if the photomask is excellent with its main surface shape having high flatness before chucking, when the photomask is chucked on the mask stage of the exposure apparatus, the photomask is deformed depending on the affinity with the mask stage or the vacuum chuck so that the flatness thereof is largely degraded. This tendency is remarkable particularly in the case of a substrate that tends to be distorted due to low symmetry of the shape of its main surface. Thus, it is becoming necessary to consider the flatness of the photomask when it is chucked by the vacuum chuck. There has conventionally been proposed a method of selecting a mask substrate having good flatness after chucking on a mask stage of an exposure apparatus (see, e.g. Patent Document 1).